This invention relates to a method for building up the molecular weight of organopolysiloxane resins. More particularly, it concerns bodying such resins with a metal soap catalyst.
Customarily, silicone resins have been derived from organohalosilanes by a batch process wherein the product is prepared in a series of steps including hydrolysis, solvent stripping and acid control, catalyst addition, and bodying to the desired endpoint viscosity. These products can then be cured in its application usage. Since the resin-making process is carried out batchwise, the time in which a resin can be prepared is at least as long as the sum of the times for the discrete and separable steps. If any of these steps could be accelerated or omitted the overall resin-making process could be expedited. The bodying stage is often the most time consuming segment of this process, taking generally from 2 to 20 hours to complete. A sizable improvement in production rates is realized by accelerating this step alone. Recently, it has been discovered that silicone resins can be prepared from alkoxylated silanes, either as starting materials, or as prepared in situ from related organohalosilanes. These organoalkoxysilanes can now be manufactured continuously, affording large volumes of nearly acid-free starting materials. Due to the minimized tendency toward gellation in a low acid medium, a hydrolysis step involving alkoxylated silanes is essentially a worry-free system and is thus, a good candidate for a continuous process as well. In either case, time is still saved by omitting the acid reduction step. With both starting materials and hydrolyzed materials being supplied at a faster rate, the already identified bodying bottleneck can be expected to worsen. Thus, a rapidly bodying catalyst is needed now and is even more necessary with the advent of continuous processing. The process of the present invention provides rapidly bodied resins which avoid the above described disadvantages as well as those described in the prior art.
Several methods have been available in the prior art, for building up the viscosities of silanol-containing organopolysiloxane resins in a controlled manner. This process, known as "bodying," is designed to increase the molecular weight of organopolysiloxane hydrolyzates to make them more valuable in resin applications. During bodying, condensation of silanol radicals are effected in the hydrolyzate to lengthen the shelf life and decrease the cure time. If bodying is not carried out in a well-controlled fashion, however, premature gellation of the organopolysiloxane resin will occur, and cause total loss of the batch.
Britton, et al., U.S. Pat. No. 2,460,805, teach that organopolysiloxane polymers can be bodied with acid activated clays, such as bentonite and other hydrous aluminum silicates. Activation of the clay is accomplished by heat treatment with strong acids, namely sulfuric acid and phosphoric acid. On the other hand, R. N. Meals and F. N. Lewis, "Silicones," Reinhold Publishing Co., (1959), page 134, disclose that bodying can be carried out with metal soap catalysts, such as zinc octoate.
Both methods have disadvantages in practice, however. Acid activated clays appear to function best only if the bodying temperature is raised to the order of 200.degree. C. at which temperature process control becomes difficult--the more useful solvents boil well below this, too; and, with either acid clay bodying or metal soap catalyzed bodying, the shelf life at 25.degree. C. is often less than 3 months.
Other methods for bodying such resins are disclosed by Merrill, U.S. Pat. Nos. 3,375,223, 3,865,766 and 4,056,492 which are assigned to the assignee of the present application, and incorporated herein by reference. In these methods the silanol-containing resin is heated in the presence of a hydrogen chloride activated particulated siliceous material, e.g., diatomaceous earth and optionally a very low level of metal soap catalyst, and the bodying is carried out efficiently at temperatures substantially lower than that required with acid activated clays. Moreover, because the level of metal soap catalyst used is very low, the bodied resin is not contaminated with metal ions which, in high amounts, can impair the stability of the cured resin, as well as substantially reduce its shelf life. The drawback to this method of bodying is in the lengthy extraction by filtration of large volumes of the particulated siliceous material and the associated inefficient use of kettle volume.
The present invention provides a process wherein certain aggressive bodying catalysts can be utilized to partially condense silanol-containing organopolysiloxane resins is a controlled manner. Lengthy filtration is not necessary.
Group IV A metal soaps such as stannous octoate have been known as aggressive condensation catalysts. They are used in the industry as cure catalysts primarily in rubbers, and in Room Temperature Vulcanizable (RTV) products wherein essentially complete condensation is required. Prior to the present invention group IV A metal soaps have not been used to body or partially condense silanols. The reason for this lies in the heretofore lack of control over these aggressive catalysts. Typically, group IV A soaps such as stannous octoate or plumbous octoate remain active at room temperature, slowly increasing the viscosity of the resin beyond the desired end point. The lack of adequate shelf-life can render a resin useless for its intended purpose, and may relegate it to waste entirely.
In response to the need for an accelerated bodying step, a quenching technique was discovered which allows the use of rapid stannous soap condensation catalysts for silicone resin bodying. It was observed, for instance, that metal soaps such as zinc octoate or manganese neodecanoate effectively and rapidly quench the extremely active stannous octoate. Octanoic acid and its various salts such as potassium or tetra-n-butyl ammonium octanoate effect similar quenching of stannous octoate. Using this quenching system, bodied resins which would otherwise have gelled in less than two days at room temperature showed no further build up in their viscosity upon quenching while curing and performing normally in a final application such as for paint vehicals, electrical junction coatings, etc.
Experiments readily attest to the fact that bodying with Sn.sup.II octoate can be effectively quenched. The use of Sn.sup.II octoate for bodying allows either greatly reduced process times or temperatures or both. Savings can result from higher throughput and possibly continuous bodying, or from a reduction in energy consumption afforded by bodying temperatures which are substantially lower than reflux. Other tin catalysts such as stannous stearate and dibutyltin dilaurate are less rapid and are less attractive catalysts.
Furthermore, it has been found that bodying with stannous octoate and quenching the catalyst in accordance with the process of the present invention ordinarily results in lower levels of undesirable benzene byproducts. This is because the low acid levels of the present process offer fewer opportunities for phenyl cleavage.
It is therefore an object of the present invention to provide a process for rapidly bodying silanol-containing organopolysiloxane resins. Another object is to controllably quench the rapidly bodied resin so as to produce a resin of desired viscosity.
Another object is to provide a process utilizing an aggressive stannous soap or salt catalyst to produce the desired resin products.
Another object is to provide a process which produces the same resin products as those produced by processes requiring much longer bodying periods.
These and other objects will become apparent to those skilled in the art upon careful consideration of the accompanying specification and claims.